1. Field of the Invention
The present invention relates to a phase change memory cell with tubular heater and to a manufacturing method thereof.
2. Description of the Related Art
As is known, phase change memories use a class of materials that have the property of switching between two phases having distinct electrical characteristics, associated with two different crystallographic structures of the material, and precisely an amorphous, disorderly phase and a crystalline or polycrystalline, orderly phase. The two phases are hence associated with resistivities of considerably different values.
Currently, the alloys of elements of group VI of the periodic table, such as Te or Se, referred to as chalcogenides or chalcogenic materials, can be used advantageously in phase change memory cells. The currently most promising chalcogenide is formed from an alloy of Ge, Sb and Te (Ge2Sb2Te5), which is now widely used for storing information on overwritable disks and has been also proposed for mass storage.
In the chalcogenides, the resistivity varies by two or more orders of magnitude when the material passes from the amorphous (more resistive) phase to the crystalline (more conductive) phase, and vice versa.
Phase change can be obtained by locally increasing the temperature. Below 150° C., both phases are stable. Starting from an amorphous state, and rising the temperature above 200° C., there is a rapid nucleation of the crystallites and, if the material is kept at the crystallization temperature for a sufficiently long time, it undergoes a phase change and becomes crystalline. To bring the chalcogenide back to the amorphous state it is necessary to raise the temperature above the melting temperature (approximately 600° C.) and then rapidly cool off the chalcogenide.
Memory devices exploiting the properties of chalcogenic material (also called phase change memory devices) have been already proposed.
In a phase change memory including chalcogenic elements as storage elements, a plurality of memory cells are arranged in rows and columns to form an array. Each memory cell is coupled to a respective selection element, which may be implemented by any switching device, such as a PN diode, a bipolar junction transistor or a MOS transistor, and includes a chalcogenic region of a chalcogenide material in contact with a resistive electrode, also called heater. A storage element is formed at a contact area between the chalcogenide region and the heater. The heater is connected to a conduction terminal of the selection element.
In fact, from an electrical point of view, the crystallization temperature and the melting temperature are obtained by causing an electric current to flow through the resistive electrode in contact or close proximity with the chalcogenic material and thus heating the chalcogenic material by Joule effect.
In particular, when the chalcogenic material is in the amorphous, high resistivity state (also called the reset state), it is necessary to apply a voltage/current pulse of a suitable length and amplitude and allow the chalcogenic material to cool slowly. In this condition, the chalcogenic material changes its state and switches from a high resistivity to a low resistivity state (also called the set state).
Vice versa, when the chalcogenic material is in the set state, it is necessary to apply a voltage/current pulse of suitable length and high amplitude so as to cause the chalcogenic material to switch to the amorphous phase.
Two types of heaters are mostly used in phase change memory devices.
So-called “lance” heaters include resistive rods having first ends in contact with the chalcogenic material and second ends connected to conduction terminals of respective selection elements. Lance heaters are made by firstly opening holes in a dielectric layer covering the selection elements, at locations corresponding to conduction terminals thereof. The holes are narrowed by formation of spacers, until a sublithographic cross-dimension is reached, and then filled with a resistive material. After a CMP (Chemical-Mechanical-Polishing) step, a chalcogenic layer of a chalcogenic material is deposited and defined, to form chalcogenic regions in contact with respective lance heaters. Accordingly, manufacture of lance heaters is very simple since only conventional process steps are involved. Despite sublithographic cross-dimensions, however, the contact areas between the lance heaters and the chalcogenic regions are quite large. In particular, minimum obtainable cross-dimension is limited by the precision in controlling the thickness of spacers formed in a deep hole. In practice, it is very difficult to form lance heaters having cross-dimensions less than 50-60 nm using a 90 nm technology. Thus, rather high currents are required to provide sufficient heating of the chalcogenic material and to cause phase transitions.
According to U.S. Pat. No. 6,816,404, to reduce the amount of current needed to cause the chalcogenic material to change its state, a “wall” heater is formed by a wall structure obtained by depositing a layer, having sublithographic thickness, of a suitable resistive material. Furthermore, the chalcogenic material includes a thin portion extending transversely to the wall structure, so as to obtain a small contact area. Wall heaters, however, are more complicated and expensive to build than lance heaters. In particular, additional masking steps are required to form the thin chalcogenic portions.